Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such devices are typically powered by self-contained energy sources, such as batteries. It is useful to know how much energy capacity remains in a battery carried within an implanted medical device, such as to ascertain when the implanted device should be explanted from the subject and replaced by a device with a fresh battery. Determining how much energy is left in a battery is particularly difficult when a measured battery characteristic (e.g., the quiescent voltage at the battery terminals) does not change appreciably during a large portion of the battery life. Yet such a characteristic is particularly desirable for use in a cardiac rhythm management device because it provides a predictable battery characteristic during that large portion of the battery life.